Receiving more than Data - A Signal Model and Theory of a Cognitive IEEE 802.15.4 Receiver(Tim Esemann, Horst Hellbrück), In Proceedings of the 10th International Conference on Cognitive Radio Oriented Wireless Networks (CROWNCOM), 2015. [bib][abstract]

In standard medium access, transmitters perform spectrum sensing. Information about concurrent interferers is gained mainly during this sensing period.Especially during transmission respectively reception there is a blind gap where transmitter and receiver have limited capabilities to detect interferer. Standard radio receiver devices for IEEE 802.15.4 provide solely data output and no cognitive capabilities. Particularly mobile interferer create problems when moving gradually into reception range. First, they create small interference before actually causing collision later, when approaching. However, small interference is not yet detectable by today?s transceivers. As a solution, we provide a signal model and an architecture for an extended cognitive IEEE 802.15.4 receiver as a basis for advanced signal processing for interference detection. The results of our theoretical analysis verify that the received signal contains signal marks of the interferer and therefore holds more information than transmitted data. Our theory is evaluated by simulations and experiments with a pair of IEEE 802.15.4 transmitter and an extended cognitive receiver.

With the increasing number of wireless devices and competing technologies in overlapping frequency bands an improved spectrum sensing is needed. Todays IEEE 802.15.4 transceiver perform spectrum sensing before transmission. They cannot detect and classify interference during transmission and reception reliably as there is a blind gap during transmission. Strong interference can be detected if bit errors occur. However, the transmission time is a long time interval compared to spectrum/carrier sensing before. In a crowded 2.4GHz ISM band with mobile interfering sources transceivers need continuous spectrum awareness to improve coexistence. In our approach we extend the physical layer (PHY) of a traditional receiver with additional signal processing. We perform signal analysis with Fourier transform of a demodulated signal to detect interferer during reception before bit errors or collisions occur. We implement our algorithm with a software defined radio as an extension of the physical layer (PHY) of an IEEE 802.15.4 transceiver. We evaluate our approach with measurements and detect mobile interferers in a real world environment. We discuss the design space of solutions as a trade-off between the quality of interference detection and the time for detection.

Some medical applications require precise information of position and orientation of a patient as changes affect pressure condition inside the body. In this paper we focus on altitude estimation, where altitude is a distance, in vertical direction, between a reference and a point of a human body. We suggest equipping wireless sensor nodes with high resolution pressuresensors to calculate the altitude with the barometric formula. We implement a body sensor network based on IEEE 802.15.4 and synchronization mechanism with a reference. Pressure variations due to environmental effects are compensated by cancellation with this differential measurement setup. We demonstrate the need for differential measurements and show with a series of measurements that environmental pressure variations have no significant effect on the proposed altitude estimation. Compared to existing systems, our solution is cost effective, easy to deploy and provides a flexible tradeoff between precision and location lag by adjusting a filter constant.

Spectrum sensing is a major task for wireless devices in order to improve coexistence among them in heterogeneous radio environments. Wireless communication includes at least two partners: transmitter and receiver. Therefore, cooperation between partners can improve the performance of spectrum sensing by reducing effort, improving sensing result or a combination of both. An optimal cooperative sensing scheme is a first step to achieve complete awareness of the radio environment for wireless devices. To the best of our knowledge, this is the first theoretical work performed in order to understand the problem of developing optimal cooperative sensing schemes for heterogeneous radio environments for multiple users and single channel. We analyze the problem and perform analytical work which results in a cooperative sensing model. The model comprises sensing schedule, data fusion rules, PU's traffic pattern, and detection performance of the sensing device. A new performance evaluation metric is introduced for optimum spectrum sensing in heterogeneous radio environments. An evaluation of available exemplary cooperative sensing schemes shows that none provides optimality in all scenarios.

Some applications in Sensor Networks need firm real-time support in order to work properly. The difference to hard real-time systems is that this type of application can withstand minor violations of the maximum delay and minimum throughput if these violations are limited. Many standards like IEEE 802.15.4 provide standardized means to ensure delay and bandwidth constraints which work well when there are no interferers in the same frequency band. However, in a heterogeneous environment today these approaches fail when the interference is not aware of the IEEE 802.15.4 traffic. Switching the channel is one option to avoid this kind of interference. We suggest a new non-invasive cognitive radio protocol approach where all participants follow simple rules to enable firm real-time conditions in decentralized design. As a demonstrator we use a three-fold pendulum with firm real-time signal delay constraints of 5ms. The contributions of the paper comprise evaluation results by real measurements with the demonstrator system.

[2011]

CSOR - Carrier Sensing On Reception(Tim Esemann, Horst Hellbrück), In Proceedings of the 4th International Conference on Cognitive Radio and Advanced Spectrum Management (CogART), 2011. [bib][pdf][abstract]

Since the 1990s the number of wireless devices increases and new areas of applications evolved. Therefore, frequency spectrum has become a scarce resource with no free frequencies left all over the world and interference between transmissions sharing the same frequency band is started to become one of the major problems in wireless transmission. The ISM bands become crowded with various standards sharing the same frequency band. One solution to the problem is to use frequency bands that are rarely used by the licensed users like TV channels, where in some region specific channels are not used at all. As a result, we need to develop adaptive systems that search for unused spectrum, use it as long as the band is free and shift to other frequency bands if there is a risk to interfere with a primary user. Such systems being aware of their radio environment are called Cognitive Radios. To search for unused frequencies and detect primary users approaches that listen before talk or efficient carrier/spectrum sensing algorithms have been presented in the past. These mechanisms are incomplete and one of the drawbacks of today's wireless transmissions is that communication partners do not detect interference reliable during an ongoing transmission. In this paper we suggest a cross layer approach Carrier Sense on Reception (CSOR) that extends the functionality of the physical layer of a transceiver to be able to detect interference while receiving data. We introduce the idea, describe the concept and give first evaluation results as a proof of concept based on real measurements.

The number of mobile devices with radio transceivers is increasing. However, standard wireless sensor nodes have limited spectrum awareness in order to avoid col- lisions with other concurrent transmissions in dense spectrum. E.g. IEEE 802.15.4 standard performs carrier sensing before start of the transmission. Spectrum sensing or awareness during the transmission is not provided. These low power devices have only limited capabilities in order to detect and forecast upcoming collisions. In this work, wireless sensor nodes are equipped with additional piggybacked hardware and supplementary signal processing capabilities. An additional RF-frontend and small-size SDR hardware enable sensor nodes to perform cognitive radio functionality. Although the sensor nodes transmit data fully com- pliant to IEEE 802.15.4 the supplementary hardware enhances spectrum awareness significantly even during transmission. A previously published cognitive radio scheme was implemented to demonstrate the signal processing capabilities of the SDR hardware. Additionally, power consumption and battery lifetime were evaluate and calculated.

[2013]

A Reusable and Extendable Testbed for Implementation and Evaluation of Cooperative Sensing(Tahir Akram, Tim Esemann, Torsten Teubler, Horst Hellbrück), In The 8th ACM International Workshop on Performance Monitoring, Measurement and Evaluation of Heterogeneous Wireless and Wired Networks PM2HW2N'13, 2013. [bib][abstract]

Cooperative sensing has been identi?ed as a potential improvement for cognitive radios to perceive their radio environment. In the past, algorithms have been developed by analysis and simulations exclusively. With cheaper hardware experimental platforms have been used for evaluation purpose recently. Simulations lack realistic propagation models for radio transmission but are reproducible compared to experimental evaluation done by hand. The effects of reduced detection probability and false alarms are not realistic in these simulations. In this paper, we suggest a reusable and extendable automated testbed software and instructions for deployment of own testbeds. Primary users as well as secondary users with cooperating cognitive radios can be flexibly deployed in the testbed within seconds. The advantage is that a series of even long lasting measurements including automatic logging of results can be easily repeated. Results can be assessed on the fly during the ongoing evaluation by accessing debug output remotely. The testbed supports stationary, portable, and in the future mobile radio devices for flexible scenarios as well as monitoring devices for debugging. The testbed and the radio devices are validated by deploying primary and secondary user in a small scenario whose outcome was analyzed beforehand. The results are as predicted and show the usefulness of this approach.

In the field of medical applications there are special regulations and requirements as many devices are sensitive to interference by other equipment. Although wireless links are susceptible to interference they are potential technologies enabling mobile and wireless applications substituting cables in all medical areas like operating room, intensive care unit or ambient assistant living. With continuous increase of these devices equipped with radio interfaces a non-homogenous radio environment with dense occupancy will form. We propose an approach with high sensitivity for non-invasive cognition of radio environment by fusion of two baseband processing blocks. First, we detect and identify interference by concurrent radio links non-invasively during reception. Second, we minimize invasive channel switching by continually probing and classification of available channels based on adapted probabilities. We use GNU Radio a Software Defined Radio platform to implement this system to provide a non-invasive continuous cognition of the radio environment and minimize the invasive utilization of occupied channels. In this paper we present preliminary studies, the overall idea, the approach and first implementation results.

Wireless systems based on WLAN (802.11), ZigBee (802.15.4) and Bluetooth (802.15.1) are continuously deployed in new applications covering consumer, industry or medical fields. Especially, Bluetooth is recommended by the Health-Care-Organization for medical applications as frequency hopping is considered as a robust scheme. However dealing with frequency-dynamic sources of interference in the 2.4GHz ISM band is important due to the increase of wireless devices. Adaptive frequency hopping (AFH) suggested by the Bluetooth standard and implemented in many of todays products identifies and avoids using bad channels. It is a good and established coexistence mechanism in the presence of frequency-static sources of interference such as WLANs when the 2.4GHz band is not crowded. However, AFH is facing problems in a crowded 2.4GHz band, especially when the interference is dynamic. We developed a cross-layer algorithm SAFH (Smooth Adaptive Frequency Hopping) that is inspired by entropy maximization and the conventional Bluetooth AFH. SAFH assigns usage probabilities to all channels based on an exponential smoothing filter for frame error rates to estimate and predict the channel conditions. The application layer can adapt SAFH by parameter settings in a cross-layer approach. SAFH achieves low average frame error rate and responds fast to changing channel conditions if required from the application. Simulative Evaluation in the presence of different types of interference (802.11b, 802.15.4 and 802.15.1) shows that our algorithm outperforms conventional frequency hopping and AFH. Additionally, SAFH works smoothly and stable exploiting frequency diversity compared to previous approaches like entropy-maximization based adaptive frequency hopping and Utility Based Adaptive Frequency Hopping (UBAFH).

In the past a number of wireless standards evolved in industry and the consumer market that operate in the unlicensed ISM (Industrial, Scientific, Medical) Bands especially in the 2.4GHz range. Today these products are well engineered and standardized, so that more and more medical applications consider wireless transmission based on these standards. Especially in the clinical environment radio links will substitute cables and further enable new fields of applications. Also personal healthcare and ambient assisted living are emerging fields for the future. However, in contrast to many consumer applications medical applications need to guarantee interoperability, ensure coexistence with other applications and have very high requirements for robustness, safety and security. There is an upcoming need for investigation if existing radio standards are prepared for the usage in medical applications regarding reliability and interoperability. In this work we will focus on Frequency Hopping and Bluetooth as one of the technologies that has been designed for robustness and coexistence and evaluate the underlying principles by a comprehensive analysis, simulation and measurements. To the best of our knowledge this comprehensive investigation for robustness of wireless transmission based on Frequency Hopping in a heterogeneous mix of interference from standards like WLAN, IEEE 802.15.4, Bluetooth and proprietary systems in health care environment has not been performed yet. We will conclude the paper by an outlook for further improvement of wireless transmission based on Frequency Hopping suited to the needs of medical applications.